Radio Frequency IDentification (RFID) systems typically include tags and RFID readers, which are also known as RFID reader/writers. RFID systems can be used in many ways for locating and identifying objects to which they are attached. RFID systems are particularly useful in product-related and service-related industries for tracking large numbers of objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using a device called an RFID reader to interrogate one or more RFID tags. Interrogation is performed by the reader transmitting a Radio Frequency (RF) wave. A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave, a process known as backscatter. Backscatter may take place in a number of ways.
The RF wave returned from a tag may further encode data stored internally in the tag, such as a number. The response, and the data if available, is decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a power management section, a radio section, and frequently a logical section, a memory, or both. In earlier RFID tags, the power management section included a power storage device, such as a battery. RFID tags with a power storage device are known as active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered by the RF signal it receives enough to be operated. Such RFID tags do not include a power storage device, and are called passive tags.
In some RFID readers, matched filters are used to receive backscatter signals from tags. However, tags may have a relatively large timebase variation (i.e., the frequency of the backscatter signal). This variation can impact the performance of the reader's matched filter in receiving data from the tags.